1.返回值解析器介紹

返回值解析器用于解析Hanlder執(zhí)行方法后的返回結果，例如將方法上標注有@ResponseBody注解的返回值解析成JSON、將方法返回的字符串作為視圖名等

SpringMVC中默認的返回值解析器見RequestMappingHandlerAdapter#getDefaultReturnValueHandlers

private List<HandlerMethodReturnValueHandler> getDefaultReturnValueHandlers() {List<HandlerMethodReturnValueHandler> handlers = new ArrayList<>(20);handlers.add(new ModelAndViewMethodReturnValueHandler());handlers.add(new ModelMethodProcessor());handlers.add(new ViewMethodReturnValueHandler());handlers.add(new ResponseBodyEmitterReturnValueHandler(getMessageConverters(),this.reactiveAdapterRegistry, this.taskExecutor, this.contentNegotiationManager));handlers.add(new StreamingResponseBodyReturnValueHandler());handlers.add(new HttpEntityMethodProcessor(getMessageConverters(),this.contentNegotiationManager, this.requestResponseBodyAdvice));handlers.add(new HttpHeadersReturnValueHandler());handlers.add(new CallableMethodReturnValueHandler());handlers.add(new DeferredResultMethodReturnValueHandler());handlers.add(new AsyncTaskMethodReturnValueHandler(this.beanFactory));handlers.add(new ModelAttributeMethodProcessor(false));handlers.add(new RequestResponseBodyMethodProcessor(getMessageConverters(),this.contentNegotiationManager, this.requestResponseBodyAdvice));handlers.add(new ViewNameMethodReturnValueHandler());handlers.add(new MapMethodProcessor());// 添加自定義的返回值解析器if (getCustomReturnValueHandlers() != null) {handlers.addAll(getCustomReturnValueHandlers());}// 兜底的解析器if (!CollectionUtils.isEmpty(getModelAndViewResolvers())) {handlers.add(new ModelAndViewResolverMethodReturnValueHandler(getModelAndViewResolvers()));}else {handlers.add(new ModelAttributeMethodProcessor(true));}return handlers;
}

這些返回值解析器會被封裝到HandlerMethodReturnValueHandlerComposite中，處理返回值時，會按照加入的順序依次判斷每個返回值解析器能否解析此任務，如果其中某個解析器能夠解析此參數(shù)，則返回解析結果

2.自定義返回值解析器

在某些場景下，我們可以自定義返回值解析器，將響應的返回值轉成我們希望的格式，假如我們想要將響應轉換為yml格式，這時我們可以自定義注解，然后再自定義返回值解析器實現(xiàn)HandlerMethodReturnValueHandler接口，示例如下：

1）創(chuàng)建自定義注解@Yml及自定義返回值處理器YmlReturnResolver實現(xiàn)HandlerMethodReturnValueHandler接口

@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.METHOD)
public @interface Yml {
}public class YmlReturnResolver implements HandlerMethodReturnValueHandler {// 判斷返回值解析器是否支持@Overridepublic boolean supportsReturnType(MethodParameter methodParameter) {Yml yml = methodParameter.getMethodAnnotation(Yml.class);return yml != null;}@Overridepublic void handleReturnValue(Object o, MethodParameter methodParameter, ModelAndViewContainer modelAndViewContainer,NativeWebRequest nativeWebRequest) throws Exception {// 將返回值轉換為yamlString str = new Yaml().dump(o);// 設置響應內容HttpServletResponse response = nativeWebRequest.getNativeResponse(HttpServletResponse.class);response.setContentType("text/plain;charset=utf-8");response.getWriter().print(str);// 設置請求已處理完畢modelAndViewContainer.setRequestHandled(true);}
}

2）配置類中加入自定義返回值解析器

@Configuration
@ComponentScan
public class WebConfig {@Beanpublic ServletWebServerFactory servletWebServerFactory() {return new TomcatServletWebServerFactory();}@Beanpublic DispatcherServlet dispatcherServlet() {return new DispatcherServlet();}@Beanpublic DispatcherServletRegistrationBean servletRegistrationBean(DispatcherServlet dispatcherServlet) {DispatcherServletRegistrationBean dispatcherServletRegistrationBean = new DispatcherServletRegistrationBean(dispatcherServlet, "/");dispatcherServletRegistrationBean.setLoadOnStartup(1);return dispatcherServletRegistrationBean;}@Beanpublic MyRequestMappingHandlerAdapter requestMappingHandlerAdapter() {MyRequestMappingHandlerAdapter handlerAdapter = new MyRequestMappingHandlerAdapter();// 添加自定義返回值解析器handlerAdapter.setCustomReturnValueHandlers(Arrays.asList(new YmlReturnResolver()));return handlerAdapter;}
}

3）創(chuàng)建控制器類及實體類

@Controller
public class Controller01 {@GetMapping("/test")@Ymlpublic User test() {